Synthesis gas, a mixture of carbon monoxide (CO) and molecular hydrogen (H2), is a valuable industrial feedstock for the manufacture of a variety of chemicals, for example methanol and acetic acid. Synthesis gas (also referred to as syngas) can also be used to prepare higher molecular weight alcohols or aldehydes as well as higher molecular weight hydrocarbons.
Perhaps the most common commercial source of synthesis gas is the steam reforming of coal or of hydrocarbons, such as natural gas. In the steam reforming process, a mixture of water and hydrocarbon is contacted at a high temperature, for example in the range from about 850° C. to about 900° C., and typically in the presence of a catalyst to form a mixture of hydrogen and carbon monoxide. Using methane as the hydrocarbon, the theoretical stoichiometry for the steam reforming reaction is as follows:CH4+H2O->3H2+CO
The steam reforming reaction is a highly endothermic reaction, and, as discussed above, it produces a relatively high molar ratio of hydrogen to carbon monoxide.
Other methods, which are more attractive than the steam reforming reaction, are available for preparing synthesis gas. One such method is the reaction of a hydrocarbyl compound such as methane with carbon dioxide. This reaction proceeds according to the following equation:CH4+CO2->2H2+2CO
This reaction, like the steam reforming reaction, is strongly endothermic and occurs at fairly lengthy contact times of approximately 1 second or more. This reaction does, however, produce a low ratio of hydrogen to carbon monoxide (1:1) and it is very useful where there is an abundant supply of carbon dioxide, for example, at a refinery or near naturally-occurring CO2 reserves. Additionally, the reforming reaction using carbon dioxide can be used in conjunction with the steam reforming reaction to adjust the ratio of hydrogen to carbon monoxide.
Alternatively, synthesis gas can also be produced by the partial oxidation of a hydrocarbon, for example methane, producing synthesis gas having a lower ratio of hydrogen to carbon monoxide (2:1), according to the following equation:CH4+1/2O2->2H2+CO
Catalytic partial oxidation of methane can occur at shorter contact times (10−3 s or less) and it can produce synthesis gas more selectively and efficiently. Unlike the steam/CO2 reforming reactions, catalytic partial oxidation of methane is mildly exothermic and does not require a large energy input. Due to these characteristics, the preparation of synthesis gas via catalytic partial oxidation of methane can greatly reduce the required capital investment.
In all of the processes described above for preparing synthesis gas, it is advantageous for the reaction to be carried out in the presence of a catalyst. Catalysts for the steam reforming of methane and of other hydrocarbons are commonly based on nickel as the active catalyst component.
Vernon et al. [Catalysis Letters, 1990, 6:181–186] disclosed that methane can be converted to synthesis gas over catalysts such as palladium, platinum, ruthenium on alumina, nickel on alumina, and certain transition metal oxides including Pr2Ru2O7 and Eu2Ir2O7. Vernon et al. disclosed that nickel-on-alumina catalysts are effective for the conversion of methane to synthesis gas using molecular oxygen. However, such a catalyst, as well as commercial nickel-containing steam reforming and steam cracking catalysts, form coke as a by-product in amounts that lead to a relatively rapid deactivation of the catalyst. Although the other catalysts described in Vernon's paper, such as ruthenium on alumina, can be used to convert methane in the presence of molecular oxygen, such transition metals are expensive.
Choudhary et al. [Catalysis Letters, 1993, 22(4):289–297; Catalysis Letters, 1992, 15(4):363–370] disclosed that alkaline and rare earth oxide supported nickel catalysts (Ni loading greater than 10 wt %) were capable of providing a 91% CH4 conversion, a 95% H2 selectivity and a H2:CO ratio of 2:1 in a synthesis gas product, using a pure methane:pure oxygen molar ratio of 1.8:1 at a contact time of 4.8 ms. Choudhary et al. also disclosed [Catalysis Letters, 1995, 32(3,4):387–390; Journal of Catalysis, 1997, 172:281–297] that supported nickel catalysts prepared by using commercially sintered, low-surface area porous catalyst carriers (e.g. SiO2 and/or Al2O3) precoated with MgO, CaO or rare-earth oxide show higher activity, selectivity and productivity in methane-to-syngas conversion reactions, than the catalysts prepared using catalyst carriers without any precoating.
Lu et al. [Journal of Catalysis, 1998, 177:386–388] disclosed a CaAl2O4-modified-Al2O3 supported nickel catalyst with a Ni-loading of 2.9% by weight used for partial oxidation of methane. Such a catalyst offers approximately 80% CH4 conversion and approximately 93% H2 and approximately 90% CO selectivity during 100-hour running at 600° C. and a contact time of 4 ms.
U.S. Pat. No. 3,791,993 to Rostrup-Nielsen discloses the preparation of catalysts containing nickel for reforming gaseous or vaporizable liquid hydrocarbons using steam, carbon oxide, oxygen and/or air. The catalysts disclosed therein are prepared by co-precipitating a nickel salt, a magnesium salt and an aluminate to form a sludge, washing the sludge until it is substantially free of sodium and potassium, drying, and then dehydrating at 300–750° C. The catalyst in its final form is obtained after a calcination step at 850–1100° C. The examples in U.S. Pat. No. 3,791,993 show that compositions having a 1:1:2 or a 2:7:1 mole ratio of nickel, magnesium and aluminum, respectively, are suitable for converting naphtha to hydrogen-rich gaseous products using steam reforming.
U.S. Pat. No. 6,271,170 to Suh discloses the preparation of catalysts containing nickel and alumina aerogel which are used for the carbon dioxide reforming of methane to prepare synthesis gas. The catalysts disclosed therein are prepared by a sol-gel method and supercritical drying, followed by an initial thermal treatment in an inert atmosphere at 200–500° C. and a secondary thermal treatment at a temperature higher than 500° C. in air or oxygen.
U.S. Pat. No. 6,242,380 to Park discloses the process for preparing a supported nickel catalyst for reforming hydrocarbons using steam, carbon dioxide, and oxygen. The catalyst disclosed therein is prepared by mixing a nickel salt, an alkali metal salt and/or alkaline earth metal salt with a silicon and/or aluminum-containing support, such as a zeolite, silica or alumina, decomposing the metal salts while melting all the salts, and calcining the decomposed metals at 300–1200° C. The examples in U.S. Pat. No. 6,242,380 show that a pentasil-type ZSM-5 (molar ratio of silicon/aluminum is greater than 500) supported K—Ni—Ca catalyst with a 0.08:1:3.2 mole ratio of potassium, nickel and calcium respectively, is suitable for converting CH4 to synthesis gas via CO2 reforming.
U.S. Pat. No. 5,653,774 to Bhattacharyya discloses the preparation of a nickel containing catalyst for preparing synthesis gas by reacting a hydrocarbyl feed material with a source of oxygen. The catalysts disclosed therein are prepared by thermally activating a nickel-containing catalyst precursor compound having a structure that is referred to as “hydrotalcite-like” at 700° C. or higher. The examples in U.S. Pat. No. 5,653,774 show that the catalysts derived from NiMg5Al2(OH)16CO3, NiMg3Al2(OH)12CO3, Ni2Mg2Al2(OH)12CO3, Ni2Al4(OH)12CO3, Ni6Al2(OH)16CO3, Ni8Al2(OH)20CO3, Cu2Ni2Al2(OH)12CO3, or NiAl double hydroxide, are suitable for preparing synthesis gas via partial oxidation of methane.
U.S. Pat. No. 4,877,550 to Goetsch discloses the preparation of synthesis gas from light hydrocarbons, e.g. methane, at elevated temperatures and pressures in the presence of a particulate catalyst, e.g. Ni/Al2O3. The example in U.S. Pat. No. 4,877,550 is demonstrated in a fluid bed reactor containing a Ni/Al2O3 catalyst operating at 982° C. and 25 atm, using a mixture of CH4:H2O:O2 with a mole ratio of 1.0:0.5:0.5 as feed gas. The synthesis gas leaving the reactor is essentially at equilibrium.
In view of the great commercial interest in preparing synthesis gas by partially oxidizing readily available hydrocarbon feedstocks such as natural gas, and because of the benefits of conducting the partial oxidation of natural gas in the presence of a catalyst that remains active for an extended period of use, there is a continuing need for new, less expensive, low metal loading, durable, coke resistant, more active and selective catalysts for the production of synthesis gas.